The US president told reporters that he, not his lawyers, wrote the answers which have not been submitted as yet. The questions are part of an ongoing investigation into possible ties between Russia and the Trump campaign.

President Donald Trump speaks during a signing ceremony of the "Cybersecurity and Infrastructure Security Agency Act" in the Oval Office of the White House in Washington. November 16, 2018.
(Evan Vucci / AP)

President Donald Trump says he has answered written questions from special counsel Robert Mueller but hasn't yet submitted them.

Trump told reporters in the Oval Office on Friday that he answered the questions "very easily" this week but added that "you have to always be careful."

The president did not say when he would turn over the answers to Mueller as part of the ongoing investigation into possible ties between Russia and the Trump campaign.

Mueller had signalled a willingness to accept written answers on matters of collusion. The White House has said it would not answer Mueller's questions on possible obstruction of justice.

Trump had huddled with his lawyer at the White House this week.

"My lawyers don't write answers. I write answers. I was asked a series of questions. I've answered them very easily," Trump said.

"I'm sure they're tricked-up because, you know, they like to catch people," Trump added, referring to questions that he suggested could be designed to result in perjury charges. "You have to always be careful when you answer, with people that probably have bad intentions. Now, the questions were very routinely answered by me."

Trump and his lawyers had been in negotiations with Mueller's team over how the president would be questioned as part of the investigation, including whether he would appear in person.

Tensions have risen even higher over the probe since Trump last week ousted Jeff Sessions as US attorney general and named Matthew Whitaker as his replacement on an acting basis.

That move gave Whitaker, a Trump loyalist, oversight of the Mueller probe rather than Rod Rosenstein, the department's No 2 official who previously had authority over it.

What is the Russia investigation?

Mueller, who Rosenstein named to head the Russia investigation in May 2017, is investigating whether members of Trump's campaign conspired with Moscow in the 2016 election and whether Trump has unlawfully sought to obstruct the probe, which has cast a cloud over his presidency.

Mueller already has brought charges against a series of former Trump aides, including his former campaign chairman and a former US national security adviser, as well as a number of Russian individuals and entities.

A federal judge on Thursday refused to dismiss criminal charges against a Russian company, Concord Management and Consulting LLC, accused by Mueller of funding a propaganda operation to sway the 2016 election in Trump's favour.

Trump on Friday again criticised the probe as a "witch hunt" and said there was no such collusion.Russia has also denied any interference.

Democrats and a number of Republicans have raised concerns about Whitaker's appointment, with a bipartisan group of US senators renewing a push for legislation to protect the special counsel. But Senate Majority Leader Mitch McConnell, a Republican, has opposed any such legislation.

Republican US Senator Lindsey Graham, who has said he would seek to lead the Senate panel overseeing the Justice Department next year, on Thursday met with Whitaker and expressed confidence the Russia probe would continue.